Embodiments of the present invention relate generally to the field of ultrasonic detection, and more particularly to a method and device for eliminating background noises in shear waves and an ultrasonic imaging system comprising the device.
Ultrasonic detection is a common detecting means in the modern medical field.
When sound waves are transmitted in a heterogeneous medium, an acoustic radiation force is produced. The acoustic radiation force can characterize bio-mechanical elasticity of tissues, which arouses interests in research on the fields of material science and medical diagnosis.
Over a long time period, one of the objectives for diagnostic imaging is precise characterization of tissues. Clinicians wish to obtain diagnostic regions of human body organs, and employ an imaging system to identify features of the tissues in the images. Ideally, clinicians expect an imaging system to identify whether a pathological state is malignant or benign. Although more efforts remain to be made to fulfill the above objective, diagnostic imaging can still reveal clues concerning the composition of tissues to clinicians. One of the technologies related to the field is elasticity imaging for use in measuring elasticity or hardness degree of tissues in a human body. For example, a highly hard breast tumor or lump may be malignant, whereas a relatively soft and more flexible lump may be benign. Owing to the relevancy between the hardness degree of a lump and its malignancy or benignancy, elasticity imaging offers clinicians another evidence useful for conducting diagnosis and working out relevant treatment solutions.
One solution is to measure shear waves. When compression on a certain point in a human body is released, the lower-strata tissues are compressed downwards and then rebound upwards during release of the compression force. Meanwhile, the tissues under the compression force link to the surrounding tissues in so continuous a way that the uncompressed tissues located at the side direction of the force vector will also make a response to the upward and downward movement of the compressed tissues. This dimple effect in the side direction is also referred to as shear waves, which serve as a response made by the surrounding tissues to the downward compression force. Further, it has been determined that the force required to push the tissues downwards can be produced by radiation pressure from ultrasonic pulses, and moreover, ultrasounds can be used to receive, perceive and measure tissue movement induced by the shear waves. The shear wave velocity is determined by mechanical properties of the local tissues. The shear waves pass through the soft tissues at a certain speed, and pass through the hard tissues at a higher speed. By measuring velocity of the shear waves at a certain point in the body, information concerning properties of the relevant tissues can be obtained, such as shear elasticity modulus, Young's modulus and dynamic shear viscosity of the tissues. The shear waves transmitted in the side direction are transmitted slowly, usually at several meters per second or even less, such that the shear waves can easily be detected, but the shear waves will quickly attenuate in a distance of several centimeters or less. Since it is allowed to repeat a same “push pulse” for each measurement, shear wave technology aids in objectively quantifying tissue properties by means of ultrasounds. In addition, the shear wave velocity is independent from the push pulse strength, thereby lessening dependency of measurement on users.
Acoustic Radiation Force Imaging (ARFI) adopts focus ultrasounds to bring radiation forces to little tissue volume. Then, conventional ultrasounds are used to track shear wave displacement of tissues, and elasticity of the tissues is calculated via an ultrasound-based relevant method. Images from the ARFI method provide mechanical impulse response information of the tissues. With respect to B mode (black-white mode), the ARFI method can improve the contrast ratio, especially when it is applied to lungs and chests. When tissues undergo pathological changes, the elasticity may change markedly. Much clinical research demonstrates that information about elasticity nature of tissues is essential to diagnosis and treatment of cancers. ARFI image, complementary to B mode image, is a method useful for characterizing mechanical characteristics of tissues.
Nevertheless, ARFI also has its own weaknesses. Its main weakness lies in that the amplitude of shear waves is extremely low and is only around 10 μm such that AFRI is sensitive to such noises as system electronic noises and a patient's movement or the like, particularly to cardiac movement or respiratory movement, which will affect estimation over the shear wave displacement. In the prior art, a solution to the aforesaid problem is to increase wave beam transmission voltage or its duration. Nonetheless, the solution will remarkably increase sound output power, even to an extent of exceeding the limitations imposed by Food and Drug Administration (FDA).
The objective of embodiments of the present invention is just to address the above problem.